THETA ANT (Theta Antliae). As modern constellations go, Antlia (the Air Pump), which lies on the
southern border of central Hydra, seems
especially obscure. Even its luminary (Alpha Antliae) shines at but fourth
magnitude (4.25), the rest of its Greek-named stars falling into fifth
and even sixth. Not that there are many of them, just Alpha (a K4
giant), Delta, Epsilon, and five others, with Beta and Gamma gone
missing. Of them, fifth magnitude (4.9) Theta Antliae ranks fourth
in brightness. Its various distinctions are that it is the most
northerly of the set, that it is a close double with a rather decently
observed orbit, and that much we seem to know about the binary is
confusing, if not downright conflicting.

The best solution to the data shows that Theta Antlia B orbits
Theta A every 18.29 years at an average separation of 11.45
Astronomical Units. The closeness of the pair (note the scale),
however, results in large observational uncertainty. An
eccentricty of 0.40 allows the stars to be as far apart as 16.1 AU
and as close as 6.8, which is still significantly farther than
Jupiter is from the Sun (5.2 AU). The offsets of the semi-major
axis and focus of the ellipse is the result of an orbital tilt of
53 degrees to the plane of the sky and the orbital orientation. In
reality the two stars have mutual orbits about a common center of
mass. From the Sixth Catalog of Orbits of Visual Binary Stars
, W. I. Hartkopf and B. D. Mason, US Naval Observatory Double
Star Catalog, 2006.

Since Theta A and Theta B are but a couple tenths of a second of
arc apart, their magnitudes are difficult to assess, especially for
the latter, which has been made out to be as bright as 5.6. From
the Washington Double Star Catalog, we adopt 5.3 and 6.2 for
"A" and "B" respectively. The spectral classes are in no better shape.
Estimates for "A" range from A7 to F7 (not surprising if it is a
metallic-line dwarf with odd abundances, as suggested), those for
"B" from an F7 bright giant to F8 dwarf to G8 giant. We'll nominally make them
an A7 dwarf plus "F-G-something." The latter being a giant though
would make little sense, as it should then be the brighter star,
not the fainter. There are neither temperature measurements nor
simple assessments from spectral class. But at least in the range
of 6000 to 8000 Kelvin there is little correction to be made for infrared or ultraviolet light,
so we can ignore the matter. At least the distance of 339 light
years (give or take 16) is in better shape. From that we find a
luminosity of about 65 Suns for "A" and
around 30 for "B," which from theory lead to masses of roughly 2.5
Suns for the brighter, just under 2 Suns for the fainter, and
suggest that "B" is indeed a giant (as might be "A"!). With a
higher mass, "A" should be farther along in ageing, but seems not
to be. Such can happen for a close binary, as for example Algol, but Theta's stars are not all that
tightly set together to produce the needed tidal interaction. As
best we know, the two stars go around each other every 18.27 years
at an average distance between them of 11.5 Astronomical Units. A
fairly high eccentricity takes them between 7 and 16 AU from each
other. They were farthest apart in 2011. Kepler's laws then give
a combined mass of 6.3 times that of the Sun, higher than the poor
value from theory but given all the uncertainties, actually not so
bad. If there is one binary crying in the wilderness for a close
study, however, it's this one, which perhaps is something we should
add to it's various distinctions.